Because I just got an m43 hahah. I didn't realize that it wasn't 3:2 until researching into the cameras and I was originally going to just shoot it at 3:2 but then I thought that I should just learn 4:3 composition. (not that it really makes a difference to me 🙂 ).
3:2 just seemed standard in photography.
1:1 (6x6) made sense because you don't need to crop and then 645/6x7 made sense too dealing with the number of frames...I didn't think that the ratios had any particular meaning.
I didn't think about printing paper, tv's, movies, and etc. I guess other ratios probably existed beforehand and do exist for other reasons.